This proposal responds to PA-05-093: Secondary Data Analyses Based on the NICHD Study of Early Child Care and Youth Development. This study will increase understanding of factors and mechanisms that influence changes in the physical well-being of contemporary youth that are associated with forming a habit of physical activity participation. The Specific Aims of the study are (1) to examine the relation between early profiles of physical activity and the subsequent profiles of physical activity;(2) to model the relation between change in physical activity patterns over time and children's health and well-being, including BMI, weight-for-height development, body image and general health;and (3) to identify environmental predictors of the development of healthy habits of physical activity and other health outcomes, including parental encouragement, opportunities for physical activity in multiple contexts (PE class, after school activities, organized sports), and competing alternatives (e.g., TV and computer use). Using the NICHD SECCYD datasets, we propose to test specific hypotheses regarding the development and correlates of physical activity, within the social and physical contexts in which children are embedded. We will use a structural equation modeling approach that utilizes latent variable growth modeling and latent variable mixture modeling to test our hypotheses. During Year 1, we will fit models of children's physical activity, health, and social and physical environmental contexts to test hypotheses 1-3. During Year 2, we will finalize the analysis models, conduct generalizability tests, and prepare and submit for peer review multiple papers on the predictive relationship between physical activity in the early education grades and physical activity and habit formation in the later elementary grades. Current research shows that growing numbers of children and adolescents lead sedentary lives and are likely to grow up to be sedentary adults. The results of this study will inform the development of practices within school and out-of-school time settings that support children and adolescents to develop sustained habits of physical activity. The National Institute of Child Health &Human Development is committed to deeply understanding how variation in child experiences, families, child care settings, and schools link to the healthy development of children. An analysis of longitudinal data such as the SECCYD provides a complex view of child development across time and different contexts. This proposed study of the factors and mechanisms that influence changes in the physical activity and well-being of youth over time can make a critical contribution to our understanding and practice of youth engagement in physical activity, and specifically inform youth development, afterschool, and child health professionals in their work to support children in achieving their full potential for healthy and productive lives.